Name: kekule’s benzene model and its limits
Uploaded: 2023-04-30T10:30:54.000Z
Duration: 1 min 7 s
Channel: JoVE

Im Jahr 1865 schlug August Kekulé die erste definitive Struktur für Benzol vor.
Er schlug vor, dass Benzol eine zyklische Struktur aus sechs Kohlenstoffatomen mit abwechselnden Einfach- und Doppelbindungen hat. Jedes Kohlenstoffatom ist an ein Wasserstoffatom gebunden.
Dieses Modell erfüllte die Tetravalenz von Kohlenstoff, so dass alle Wasserstoffatome gleichwertig sind.
Kekulés Modell sagte zwei Formen von 1,2-Dibromobenzen voraus, die sich in den Positionen der Doppelbindungen unterschieden. Es wurde jedoch nur eine Form von 1,2-Dibrobbenzol isoliert.
Kekulé verteidigte sie, indem er ein schnelles Gleichgewicht zwischen den beiden untrennbaren Formen des Benzols vorschlug, die heute als Kekulé-Strukturen bekannt sind. Inzwischen weiß man jedoch, dass ein solches Gleichgewicht nicht existiert.
Darüber hinaus konnte das ungewöhnliche chemische Verhalten von Benzol, das eine Substitutionsreaktion mit Brom eingeht, nicht erklärt werden, im Gegensatz zu Alkenen, die eine Additionsreaktion durchlaufen.

struktur von benzol: kekulé-modell

Structure of Benzene: Kekul&eacute; Model

Im Jahr 1865 schlug August Kekule die Struktur von Benzol gemäß der Strukturtheorie der organischen Chemie vor, basierend auf den drei Behauptungen: Die Formel von Benzol ist C_6H_6, alle Wassersubstanzatome von Benzol sind äquivalent und jeder Kohlensubstanz muss aufgrund seiner Vierwertigkeit vier Bindungen haben.

Er schlug vor, dass Benzol eine zyklische Struktur aus sechs Kohlensubstanzatomen hat, die jeweils an ein Wassersubstanzatom gebunden sind, mit drei abwechselnden Pi-Bindungen.

<img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;">

Nach seinem Modell muss es für die 1,2-disubstituierten Benzolderivate zwei Isomere geben, wie in Abbildung 2 dargestellt, wobei in einer Struktur die beiden substituierten Kohlensubstanze durch eine Doppelbindung verbunden sind, in einer anderen jedoch durch eine Einfachbindung.

<img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;">

Diese Vorstellung von Kekule erwies sich als falsch, da für 1,2-disubstituiertes Benzol nur eine Struktur gefunden wurde. Um dies zu erklären, führte er an, dass sich die beiden Formen von Benzol in einem schnellen Gleichgewicht befänden und dass es nicht möglich sei, die beiden Verbindungen zu trennen.

<img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;">

Allerdings existiert kein solches Gleichgewicht und es wird nur eine Struktur für 1,2-disubstituierte Benzolderivate gefunden.

<img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;">

Benzol ist vierfach ungesättigt und bildet einen Ring und drei Pi-Bindungen. Im Gegensatz zu anderen ungesättigten Kohlenwassersubstanzen wie Alkenen und Alkinen geht Benzol jedoch keine Additionsreaktionen ein. Beispielsweise geht Ethylen eine Additionsreaktion mit Brom ein, um Ethylendibromid zu bilden, Benzol jedoch nicht.

<img alt="Figure5" src="/files/ftp_upload/12450/12450_PT_Figure5.svg" style="text-align: center; height: 120px;">

<img alt="Figure6" src="/files/ftp_upload/12450/12450_PT_Figure6.svg" style="text-align: center; height: 60px;">

Benzol reagiert mit Brom über eine Substitutionsreaktion in Gegenwart von Lewis-Säure-Eisenbromid unter Bildung von Brombenzol.

<img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;">

Der Hauptnachteil von Kekules Modell besteht darin, dass es diese ungewöhnliche Reaktivität von Benzol im Vergleich zu anderen ungesättigten Kohlenwassersubstanzen wie Alkenen und Alkinen nicht erklären konnte.

Struktur von Benzol: Kekulé-Modell

Im Jahr 1865 schlug August Kekule die Struktur von Benzol nach der Strukturtheorie der organischen Chemie vor, die auf den drei ...

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kekule’s benzene model and its limits

Kekule’s benzene model describes benzene as a six-carbon ring with alternating pi bonds. August Kekule proposed this structure in 1865 from the formula C6H6, the idea that all benzene hydrogens are equivalent, and the rule that each carbon has four bonds because of tetravalency.

In this model, each carbon is attached to one hydrogen atom. It also predicts two isomers for 1,2-disubstituted benzene derivatives. In one form, the substituted carbons are linked by a double bond, and in the other they are linked by a single bond.

That prediction did not match experimental results. Only one structure is found for 1,2-disubstituted benzene derivatives. Kekule tried to explain this by saying the two forms of benzene stay in rapid equilibrium and cannot be separated, but no such equilibrium exists.

Benzene has four degrees of unsaturation, which means one ring and three pi bonds. Even so, it does not undergo addition reactions the way alkenes and alkynes do. Ethylene, for example, adds bromine to form ethylene dibromide, but benzene does not.

Instead, benzene reacts with bromine by substitution when ferric bromide, a Lewis acid, is present. The main weakness of Kekule’s model is that it does not explain this unusual reactivity compared with other unsaturated hydrocarbons.

Kekule’s Benzene Model and Its Limits

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Learn how Kekule proposed benzene’s ring structure, why it predicted two isomers, and why the model could not explain benzene’s unusual reactivity.

Kekule’s Benzene Model and Its Limits | JoVE K12

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Kekule’s Benzene Model and Its Limits

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